Many microscopes are designed based on the assumption that the specimen to be examined is situated such that a transparent body (such as a cover glass or a culture dish, wall of a plastic or glass petri dish, and/or a liquid) is situated between the specimen and the microscope objective lens. Certain microscope objective lenses are designed for a particular range of thickness and refractive index of the transparent body. The design criteria include control of aberrations that could arise within the specified range of thickness and refractive index of the transparent body.
Specifications for microscopes used in biotechnology disciplines such as cell culture and genetic engineering have been changing rapidly in pace with the remarkable progress in these sciences. A popular type of microscope used in these sciences is the inverted microscope in which the specimen is viewed from underneath the dish or plate holding the specimen. Inverted microscopes require objective lenses exhibiting a long working distance (i.e., axial distance from the distal end of the objective lens to the surface of the specimen). Also, the specimen is observed through the under-surface of the dish or plate holding the specimen. Unfortunately, the thickness of glass and plastic dishes (i.e., the "transparent body" discussed above) can vary widely. Aberrations are inevitably generated by the transparent body that can cause degradations in the imaging performance of the inverted microscope.
Conventional objective lenses for use under such conditions typically have a specified (but relatively narrow) range of working distance and refractive index of the transparent body with which the lens can be used. Unfortunately, if the transparent body actually used has a refractive index and/or thickness that is out of range for the objective lens, a substantially degraded imaging performance will be evident. The magnitude of the degraded performance tends to be greater at a larger numerical aperture (NA) of the objective lens.
Certain conventional microscope objective lenses include movable lens groups inside the objective lens to compensate for variations in the thickness of the transparent body, thereby providing some correction of aberrations introduced by the transparent body. Conventional objective lenses intended for use with an inverted microscope used for examining cell-culture specimens typically have larger correction ranges for certain aberrations than objective lenses used with a non-inverted microscope. This is because cell- or tissue-culture dishes are produced in a wide variety of shapes, materials, and thicknesses, and even a population of the same type of dish from a manufacturer can vary significantly from one dish to the next.
Examples of microscope objective lenses useful for the inverted microscope are disclosed in Japanese Kokai (Laid-Open) Patent Document No. 100409 (1984) and Kokai Patent Document No. 2005521 (1985). The objective lenses disclosed in these documents comprise a movable lens group permitting manipulation of a lens spacing inside the objective lens to compensate for a range of variations in the thickness and refractive index of a wall of the culture dish. These lenses also have a comparatively large working distance and exhibit a certain correction capability for spherical aberrations. However, these lenses exhibit problems such as inferior correction of coma aberration and an unstable imaging performance around the periphery of the field of view.
Another conventional microscope objective lens is disclosed in Japanese Kokai Patent Document No. Hei 3-50517 (1991). This lens has a large numerical aperture (NA), exhibits a large working distance, and can favorably compensate for changes in aberrations caused by certain changes in the thickness of the culture dish or other transparent body. This lens comprises a second lens group that can axially move relative to the first and third lens groups for aberration correction. Unfortunately, the movable second lens group comprises a large number of lens components, making the objective lens complicated and expensive. More specifically, the second lens group is divided into multiple subgroups that are movable relative to each other. The resulting numerous degrees of freedom of movement, while useful for correcting aberrations, requires a complicated movement mechanism, which is expensive.
Other conventional microscope objective lenses are disclosed in Japanese Kokai Patent Document No. Sho 56-142508 and Japanese Kokai Patent Document No. Sho 60-260016. These lenses have a movable second lens group used for effecting a degree of aberration correction. However, the refractive power of the second lens group of the objective lens of the 56-142508 document is too strong; whenever the second group moves, the focal length of the entire objective lens changes. In other words, magnification fluctuations are evident during use of such an objective lens. The same problem exists with the objective lenses of the 60-260016 document.